1. Field of Invention
The present invention is in the field of methods for controlling instability of operation in a de-ethanizer tower in the gas recovery unit in fluid catalytic cracking units and delayed coking units.
2. Fundamentals of the Invention
Gases coming from the top section of fractionating towers of fluid catalytic cracking units and delayed coking units go through a procedure of compression, normally in two stages, and also a procedure of washing with water for removal of compounds considered to be contaminating.
The gases, now washed, are conducted, at high pressure, to an item of equipment known as a high pressure drum, for separation of water and hydrocarbons.
In terms of the operation in practice, the separation between water and hydrocarbons is subject to failures.
The immediate consequence is a presence and/or dragging of water, in a much higher quantity than foreseen by a unit's plan, together with the flow of hydrocarbons as load to a rectification tower, known as a de-ethanizer tower, in the gas recovery unit. The excess water in this de-ethanizer tower results in instability not only in its own capacity to operate, but also in the equipment connected to it.
The instability generates accumulation of water in the top of the tower, which can even reach a flooding situation. This inundation by excess liquid in the de-ethanizer tower is known by the expression “backup flood.” In serious cases, the flooding affects other equipment such as the primary absorption tower, finally causing instability of the entire gas recovery unit.
The flooding takes place, in practice, due to excess formation of steam, known by the expression “choke flood.” This excessive formation of steam takes place in the de-ethanizer tower, in the region of the upper half of the column. The excess vapor formed is the cause of the instability in the tower, in the form of its flooding.
It is this situation that leads to the need for reduction of the load of the de-ethanizer tower.
3. Description of the Related Art
In relation to the instability of de-ethanizer towers due to flooding, in the most serious case, the literature always seems puzzled by the fact that the flooding takes place in the higher part of the tower rather than the lower part, the location where there is always the largest load of liquid and, thus, the logical region for the start of a “backup flood.”
The available literature on this subject discusses cases that have happened in refineries, refers to problems relating to errors in planning, or indeed raises the question of the disparity between the projection made in computational simulators and the functioning of the equipment in practice.
Given the varying findings about diverse aspects, normally alternatives for recovering the stability of the unit are favored, in the case of problems in the gas recovery unit focusing on the de-ethanizer tower for gas recovery.
Some solutions presented in specialized technical publications can be highlighted, such as the following examples:                Reduction of the de-ethanizer tower load or parameter control (Henry Z. Kister, Component Trapping in Distillation Towers: Causes, Symptoms and Cures, CEP, August 2004, at 22-33);        Elimination of the water present in the tower by a removal procedure (Dave Langdon et al., FCC Gas Plant Stripper Capacity, PTQ REVAMPS AND OPERATIONS, 2004, at 3-7); and        Preheating of the load of the de-ethanizer tower to a temperature at which instability does not cause effects (Stephen J. Deley & Kenneth Graf, Random Packing Debottlenecks Refinery De-Ethanizing Stripper's, OIL AND GAS JOURNAL, Aug. 1, 1994, at 39-41; and Tony Barletta & Scott Fulton, Maximizing Gas Plant Capacity, PTQ REVAMPS AND TURNAROUNDS, Spring 2004, at 105-113).        
The solution above relating to the reduction of the tower load directly results in reduction of the load processed, both in the fluid catalytic cracking unit and also in the delayed coking unit.
To return the de-ethanizer tower to a stability situation, the solution relating to the removal of water present inside this de-ethanizer tower has the following proposals: installation of a device that allows outflow of the water to a vessel that is external to the tower, for accumulation and subsequent discarding; or the installation of an internal device which normally would be a plate known as a “sump” where the water is accumulated and removed from the system.
The second procedure involves the vaporization of the accumulated water before entering the de-ethanizer tower, by prior heating of the tower load to a temperature at which the water will be vaporized, that is to say, it does not enter the tower in liquid form.
The proposals for solution via removal of water and vaporization of the accumulated water have limitations. For example, for the removal of the water, the water still descends inside the tower, to be withdrawn from it only afterward.
In the prior vaporization of the water, although it avoids the water descending through the tower, if there is excessive vaporization of water, also, due to the effect of the temperature, light components in the load—such as hydrocarbons with one to four atoms of carbon—are excessively vaporized, which generates high recyclings of these light hydrocarbons to the high pressure drum with an overload of the systems involved, also affecting the primary and secondary absorbers and leading to considerable losses of hydrocarbons with three atoms of carbon in the combustible gas.
The literature, however, does not mention the occurrence of a phenomenon that takes place between the water and the hydrocarbons, which is the formation of azeotropes that are present in the load of the de-ethanizer tower. This formation is caused by water solubility in the hydrocarbon stream and also by accumulation in the high pressure drum.
The inventors realized that the azeotropy between the water and the hydrocarbons is the main factor responsible for formation of excess steam and, consequently, the basic cause of the generation of instabilities and flooding in the de-ethanizer tower, problems which until today's date have not had efficient solutions in the state of the art.